Follicle-associated epithelium (FAE) is a crucial part of the mucosal immune barrier. It contains M cells that are unique in their ability to deliver pathogens to the organized lymphoid tissues. M cells are necessary for induction of the immune response to enteral pathogens, and they also serve as entrance gates for pathogens such as Shigella, Salmonella, M. tuberculosis, and retroviruses. The current application is based on the discovery of the importance of B cells for the development of FAE and M cells, and is focused on further exploration of this relationship. The goal is to characterize the signals involved in the molecular cross-talk between B cells and epithelial cells within the FAB. The application contains two Specific Aims. Specific Aim 1 proposes to use imaging of live FAE to study the kinetics of the development of FAE and the generation and maintenance of M cells upon transfer of labeled B lymphocytes. Specific Aim 2 is designed to identify signal(s) that B lymphocytes provide to epithelial cells to drive the development of FAE and M cells. The design includes transfer of bone marrow from mice with targeted disruption of TNF-family cytokine genes into B cell-deficient recipients and testing of a hypothesis that I lateral inhibition mechanism of tissue differentiation may be involved in the generation of FAE and M cell. The significance of the proposed research program is that it aims at the identification of molecular mechanisms driving development and function of FAE. These mechanisms are critical for protection against pathogens and development of oral vaccines.